The present invention relates to differential current amplifiers, namely amplifiers capable of receiving a current I1 at a first input terminal and a current I2 at a second input terminal so as to produce at the output a current proportional to I1-I2.
It is known to construct this type of amplifier for example using a diagram such as the one shown in FIG. 1: the current I1 is applied to a resistor R1 and the current I2 to a resistor R2 of the same value as R1. The difference of the voltage drops R1I1 and R2I2 across these resistors is measured by a differential voltage amplifer 18. It is known how to construct voltage amplifiers having good rejection of the common mode. The output voltage of the amplifier is proportional to R1I1-R2I2.
If R1 is equal to R2 there is no problem. But in practice it is difficult to produce sufficiently accurate resistors, particularly if the current amplifier is to be constructed in the form of an integrated circuit. In the output signal there will appear a common mode component due to the difference in value of the resistors (component proportional to the mean value of the currents and to the difference R1-R2).
The accuracy is therefore directly linked to the pairing of the resistors.